Automatic molded case circuit breaker with time-delay overcurrent tripping

ABSTRACT

A molded case electric circuit breaker of the type including inverse-time characteristic tripping means such as a bimetallic element for releasing a latch on the occurrence of sustained moderate overload conditions and magnetic-acting means also for releasing the same latch on the occurrence of sudden highoverload conditions such as a short circuit condition; release of the latch by the magnetic-acting means occurs only after a closely controlled short time-delay period, making possible the coordination of the circuit breaker in a selectivity system with other circuit breakers, allowing time for circuit breakers closer to fault to trip.

United States Patent Pardini [451 Jan. 18, 1972 [63] Continuation of Ser. No. 721,236, Apr. 15, 1968,

abandoned.

[52] US. Cl. ..317/36 TD, 317/58 [51 Int. Cl ....H02h 3/02, H02h 3/08 [58] Field ofSearch ..335/35,40, 63;317/l4 R, 36TD,

3,211,955 10/1Q65 S og ..317/ )g 3,256,407 6/1966 Klein ..3l7/58X Primary Examiner-James D. Trammell Attorney-Robert T. Casey, Robert S. Smith, D. M. Schiller, Frank L. Neuhauser and Oscar B. Waddell [5 7] ABSTRACT A molded case electric circuit breaker of the type including inverse-time characteristic tripping means such as a bimetallic element for releasing a latch on the occurrence of sustained moderate overload conditions and magnetic-acting means also for releasing the same latch on the occurrence of sudden highoverload conditions such as a short circuit condition; release of the latch by the magnetic-acting means occurs only after a closely controlled short time-delay period, making possible the coordination of the circuit breaker in a selectivity system with other circuit breakers, allowing time for circuit breakers closer to fault to trip.

9 Claims, 9 Drawing Figures x m l u 3A 4 Q 47 4 57 are 1 o 7 11a 45 ///A Ina SHEET t 8F 4 INVENTOR. FPANtO lum/w I F|G.9

BACKGROUND OF THE INVENTION 1. Field of the Invention My invention relates to electric circuit breakers having an insulating casing commonly referred to as molded case" circuit breakers, and more particularly to circuit breakers of the type referred to including means facilitating the use of the circuit breakers in a selectively" system, that is, in a system including other circuit breakers, in which circuit breakers farthest from the source of power, sometimes referred to as downstream circuit breakers, are allowed time to trip or automatically open upon the occurrence of short circuit currents before circuit breakers closer to the source (sometimes referred to as upstream circuit breakers) trip. Such a selectivity arrangement makes it possible to deenergize only the smallest required portion of the electrical system in which the fault takes place.

2. Description of the Prior Art Molded case circuit breakers, in accordance with the prior art, have been provided with time-delay tripping on the occurrence of high short circuit currents, but in the past this has been accomplished only by the use of extremely complicated apparatus, such, for example, as hydraulic or mechanical time escapement systems or by the use of a separate time-delay over-current relay supplied by current transformers (one for each of a three-pole circuit breaker). Such apparatus is extreniely expensive and complicated and difficult to use with conventional molded case circuit breakers without major modification to the circuit breakers.

OBJECTS OF THE INVENTION It is an object of the invention to provide an electric circuit breaker of the type having long-time or inverse time current-tripping characteristics and also having short-time or high-fault current-tripping characteristics, wherein the highfault current-tripping means includes means for providing a closely controlled short time-delay between the occurrence of the high-fault current and the tripping of the circuit breaker.

, It is another object of the invention to provide an electric circuit breaker of the type described including inverse timetripping characteristic and high-fault-tripping characteristic with closely controlled time-delay which comprises a combination including'a conventional enclosed circuit breaker with a conventional auxiliary device included therein, and a closely controlled time-delaytripping means responsive to high-fault currents usable therewith without substantial modification of the conventional circuit breaker.

It is another object of the invention to provide a circuit breaker of the type described comprising a self-containedtripping unit capable of use with an essentially conventional circuit breaker and providing closely controlled time-delay on occurrence of high-fault currents, permitting the combination to be used in a selectivity distribution system.

It is still another object of the invention to provide a tripping unit of the type described in the preceding object which utilizes a substantial number of parts suitable for use in trip units of conventional prior art circuit breakers.

SUMMARY OF THE INVENTION In accordance with the invention in one form, an electric circuit breaker is provided including an insulating casing having three-pole chambers, each of the pole chambers including at least one pair of separable contacts and are extinguishing means. The casing also includes manually operable operating mechanism for moving the three sets of contacts between open and closed circuit positions simultaneously. The casing further includes inverse-time current-responsive means, such as a bimetallic strip, for each of the pole chambers and means to cause automatic opening of the mechanism upon the occur- 75 I;

rence of predetermined abnormal current conditions which are sustained for a predetermined length of time in any or all of the poles of the circuit breaker. in accordance with the invention, a second type of current responsive means is also provided for each of the three poles comprising means instantaneously responsive to high-current conditions, such as a magnet and armature for each of the pole chambers, and electrical switch means is provided which is actuated by movement of any or all of the armatures of the high-speed current-responsive means. Actuation of the switch means referred to begins the running of a controlled amount of time, at the end of which, actuation of the trip means of the circuit breaker mechanism takes place.

In accordance with one form of the invention, actuation of the switch by the high-speed trip meansremoves a source of power from a hold-in relay, which begins timing out," the length of time being determined by a capacitance-resistance combination. When the relay times out," it causes actuation of a solenoid plunger which initiates tripping of the circuit breaker mechanism. In accordance with one aspect of the invention in this form, the solenoid utilized may be a conventional prior art undervoltage release solenoid of the type commonly used in such molded case circuit breakers. It will be appreciated, however, that rather than using a device of the type which actuates upon the removal of power, I may use a solenoid which actuates upon the application of power. The undervoltage-releasetype device, is preferred, however, since by this means I achieve a fail-safe" type of operation.

In accordance with the invention in another form, actuation of the high-speed current-responsive means directly initiates actuation of a time-delay solenoid which, after the lapse of a closely controlled amount of time, causes tripping of the circuit breaker. In this form, also, a normally energized form of solenoid is preferred since it afiords fail-safe operation, although it will be appreciated that a normally deenergized type of solenoid may be utilized, whereby closing of contacts by the high-speed current-responsive means applies power to the normally deenergized solenoid, attracting its armature and thereby causing tripping of the circuit breaker after a closely controlled amount of time.

The invention will be more fully understood from the following detailed description, and the scope of the invention will be more particularly pointed out in the appended claims.

Furthermore, in accordance with the invention, the highspeed current-responsive means, together with means for providing power for the time-delay relay if used, and for capacitive-components in the time-delay system, is provided in a separate casing or auxiliary box having its cross-sectional dimensions substantially comparable to those of conventional molded case circuit breakers, and terminal means is provided for connecting this auxiliary box directly to the load end of a conventional circuit breaker of the type including inversetime current-responsive means and undervoltage trip means, but not high-speed current-responsive means.

In accordance with a still further aspect of the invention the time-delay relay utilized is provided with magnetic-shielding means which is saturable within the range of the magnetic fields produced by high short circuit currents in the main adjacent conductors. By this means, the time-delay feature is rendered unaffected by short circuit currents up to a predetermined level, but beyond such level the time-delay provided is shortened by the action of magnetic fields associated with the current in the main conductor.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevation sectional view, partly schematic, of an electric circuit breaker incorporating the invention;

FIG. 2 is a side elevation view, partly in section, showing the long-time or inverse-time current-responsive means of the circuit breaker of FIG. 1 and the undervoltage release device;

FIG. 3 is a side elevation sectional view of the high-speed, high-current responsive means of the circuit breaker of FIG.

' FIG. 4 is a plan view of the cover portion of the auxiliary enclosure of the circuit breaker of FIG. 1, as viewed from below, on the plane indicated by the line 4-4 of Fig. 1;

FIG. 5 is a plan view of a portion of the auxiliary assembly of the circuit breaker of FIG. 1 as viewed on the plane indicated by the line 5--5 of FIG. 1;

FIG. 6 is a fragmentary illustration of a portion of the auxiliary unit of the circuit breaker of FIG. 1 taken generally on the plane indicated by the line 6-6 of FIG. 5;

FIG. 7 is a top plan view of the circuit breaker of FIG. 1;

FIG. 8 is a wiring diagram of the circuit breaker of FIG, 1. and

FIG. 9 is a wiring diagram of another embodiment of the invention.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring to FIG. 1, the invention is illustrated as incorporated in an electric circuit breaker comprising a main casing portion 1 and an auxiliary casing portion 2. The circuit breaker illustrated is a three-pole circuit breaker, and the main casing 1 comprises an enclosure having three elongated parallel pole chambers. The showing of FIG. 1 is taken on a plane passing through the center pole chamber 1A, which contains the operating mechanism operated by the handle 42, to be described more fully. Also shown in FIG. 1 is an enclosed trip unit" 3, within the casing l, which extends laterally across the three-pole chambers of the circuit breaker. For convenience, the trip unit 3 is not shown in section in FIG. 1. A sectional view thereof, with details of the parts, is shown on enlarged scale in FIG. 2.

Referring to FIG. 2, a sectional view of the trip unit 3 of FIG. 1 is shown, including inverse-time current-responsive means, comprising an elongated bimetallic strip 10 supported on and heated by the heater portion 11 of a conductor connected in series in the main circuit.

The bimetallic strip 10 acts on a common trip bar by means of a calibrating screw 12. Counterclockwise rotationof the bar 13 causes collapse of the toggle linkage comprising the links 19 and 20 by engagement of the screw 17 with an 19-point of the toggle linkage. Collapse of the toggle linkage 19-20 permits clockwise rotation of the link 21 and release of the latch end 23A of the lever 23 of the circuit breaker mechanism. The lever 23 supports the toggle linkage comprising links 40 and 41, connected between the lever 23 and the contact ann portion 37. Release of the lever 23, therefore, pennits upward movement of the contact arm 37 under the force of the tension spring 43 which is connected between the knee point of the toggle levers 40-41 and anchor point 42A on the manually operable member 42. Upward movement of the contact arm portion 37 carries with it the movable contact member 37A which thereupon separates from the corresponding stationary contact 34. Although not shown, it will be uriderstood that similar contact arms are positioned in the adjacent pole chambers of the circuit breaker and are connected thereto by a common contact crossarm, not shown, which interconnects the contact arms of each of the pole chambers. The contact arms of the adjacent pole chambers pivot on an axis which is coaxial with the axis 378 of the contact arm 37.

Referring to FIG. 1, the current path through the main portion of the circuit breaker 1 may be followed from the line terminal 36A as follows: from the terminal 36A to the conductive strap 35, to the stationary contact 34, to the movable contact member 37A, to the flexible conductor 38, to the stationary conductive plate 39, to the trip unit input terminal 11A, through the heater portion 11 to the trip unit output terminal 118. The structure as thus far described is conventional. While one particular structure and mechanism has been shown and described for purposes of illustration, it will be appreciated that many different forms may be utilized in accordance with the invention. It will be observed, however, that the mechanism of the main circuit breaker portion 1, although it includes inverse-time current-responsive means, comprising the bimetallic strip 10, (and may include other current responsive devices responsive to a particular desired external signal), does not include an instantaneous-acting current-responsive means, such as a trip means of the magnetic type acting directly to cause release of the latch member 44 on the occurrence of extremely high-fault currents, such as short circuit currents.

In accordance with the invention, automatic opening of the circuit breaker 1 upon the occurrence of high-fault currents such as short-circuit currents is achieved with a closely controlled time-delay by means of the apparatus which will now be described.

For the purpose of causing release of the operating mechanism of the main circuit breaker 1 upon the occurrence of high fault currents such as short-circuit currents with a closely controlled time-delay, the main circuit breaker 1 is provided with an actuating solenoid 27, and a supplementary trip assembly 2 is provided which, in effect, controls an electric signal fed to the solenoid 27 to cause tripping when desired.

More particularly, in the embodiment illustrated, the solenoid 27 comprises a coil or winding 31 and a movable magnetic plunger 31A which, in normal operation, is biased by a compression spring 28 for movement outwardly of the coil 31 to the left as viewed in FIG. 2. The plunger 31A is provided with a rod 318 and a hook 31C which, when the plunger 31A moves to the left as viewed, engages a projection 13A on the common trip bar 13, and causes release of the retaining latch of the mechanism in a manner similar to that described in connection with the bimetallic strip 10.

SUPPLEMENTARY TRIP UNIT For the purpose of generating a control signal to initiate actuation of the solenoid 27 with closely controlled time-delay, upon the occurrence of high-fault currents such as short-circuit currents through the circuit breaker 1, there is provided, in accordance with the invention, a supplementary trip unit assembly 2 comprising an insulating casing 2A. The casing 2A preferably has its cross-sectional dimensions substantially equal to the corresponding dimensions of the main circuit breaker portion 1, as shown for example in FIG. 8 and in FIG. 1. The casing 2A includes a main input terminal 368 for each pole of the circuit breaker, a supplementary trip unit 3A, a transformer 57., and other components to be described.

The supplementary trip unit 3A is preferably of similar structure to that utilized in the main trip unit 3, and includes conductors therethrough for each pole of the three-pole circuit breaker. Refen'ing to FIG. 3, the supplementary trip unit 3A is shown as including a conductor 11] for each pole of the circuit breaker having an input terminal portion 111A and an output terminal portion 1118. The conductor 111 also includes an intennediate loop portion 1 1 1C, on one leg of which is mounted a generally U-shaped magnetic armature 14 embracing the other leg portion of the conductor 111. The conductor 111 thereby acts as an energizing winding for the magnet 14. A magnetic armature 16 is supported on a crossbar 113 pivotally supported for rotation about an axis 113A, and carrying a projection 48 attached thereto by suitable means, such as by a screw 49. The projection 48 of the bar 113 is disposed to engage an actuator 47A of a miniature switch 47 upon predetermined counterclockwise rotation of the bar 113. The switch 47 includes contacts 47B, connected in a manner to be described.

For the purpose of adjusting the pull-in point of the armature 16, a tension spring 26 is provided which is connected between a point 26A of the trip bar 113 and a projection 25A of an adjusting lever 25 pivotally supported on the insulating casing 3A. Movement of the lever 25 in the direction indicated by the arrow in FIG. 3, moves the tension spring 26 bodily in such a way that its line of action approaches more closely to the pivot axis 113A of the bar 113, thus reducing the mechanical advantage of the spring on the armature 16, reducing the effective biasing force thereon. By this means the pull-in point of the armature 16 may be adjusted. It will be understood that the supplementary trip unit 3A includes a conductor strap 111, magnet 14, and armature 16 carried by the trip bar 1 13, for each pole of the three-pole circuit breaker.

The supplementary trip aswmbly 2 also is preferably provided with a magnetically permeable plate 50 over each of the conductors 45 to shield the supplementary trip unit from the effects of magnetic current associated with high currents in the conductor 45.

Referring to FIG. 8, the electrical connection of the supplementary trip unit 3A to the main circuit breaker 1 will be described. The output portions 111-1, 111-2, and 111-3 of the conductors 111A for each of the pole chambers of the supplementary'trip unit 3A are connected respectively to input terminal portions 118-1, 113-2, and 118-3 of the trip unit 3 of the main circuit breaker, as shown in FIG. 8 and also as shown in FIG. 1. Thus the current passing through any onepole chamber of the main circuit breaker 1 passes through a corresponding conductor 111 of the supplementary trip unit 3A.

' For the purpose of providing a DC voltage for energizing the winding 31 of the undervoltage release 27, a transformer 57 is provided having its secondary or output connected to a rectifier bridge 58. The output of the rectifier 58 is connected across each of two circuit paths which are thereby connected in parallel. The first of these paths comprises the switch 47 (which is normally closed), resistor 60, and capacitance 59. The other of the paths comprises the resistor 61 and capacitance 62.

Connected in parallel with the series combination comprising resistor 60 and capacitance 59, is a relay 56 comprising winding 56A and contact 5613. The coil 31 of the solenoid 27 is connected in series with contacts 56b of the relay 56 across the output of the rectifier bridge 58 so that when the contact 56b is closed, the coil 31 is energized.

- Operation Upon the occurrence of high-fault currents, such as shortcircuit currents, through any or all of the poles of the circuit breaker 1, the armatures 16 of the supplementary trip unit 3A are attracted, rotating the actuating bar 113, and actuating (opening) the normally closed switch 47. When the switch 47 is opened, the voltage across the coil 56A of the relay 56 begins to decay at a time rate determined by the resistancecapacitance combination 59-60. While this voltage is decaying, the contacts 568 are, of course, maintained in closed condition. Therefore, the voltage applied from the rectifier S8 to the coil 31 of the actuator solenoid 27 will continue to be applied. It often happens, however, during a severe short-circuit condition, the input or line voltage E applied to the transformer 57 drops to or close to zero. Thus, if no further special provision were made, the solenoid 27 might drop out and cause premature opening of the circuit breaker 1 before the relay 56 has had time to time out and open its contacts 56. in order to guard against this possibility, the resistancecapacitance combination 61-62 is provided, connected in parallel with the output of the rectifier bridge 58. The resistor 61 and capacitor 62 are selected so as to provide a time constant sufficient to maintain voltage high enough to hold the solenoid 27 in retracted (energized) condition at least until the time-delay period for the relay 56 elapses. When this occurs, the contacts 56 open, thereby disconnecting the solenoid 27 from both the rectifier power source 58 as well as from the resistance 61-62 power source.

Referring to FIG. 9, the form of the invention illustrated therein is similar to that previously described, excepting that in this form the time-delay relay 56 is omitted, and instead, the output of the time-delay resistance-capacitance combination 59-60 is applied directly to the solenoid 27. In operation in this form, when the switch 47 is opened by the action of highfault currents on the magnetic armatures 16, the rectified voltage from the rectifier 58 is removed from the resistancecapacitance combination 59-60, and therefore its voltage begins to decay. After a predetermined length of time, determined by the size of the resistance capacitor 59 and impedance of the coil 31, the voltage drops to such a point that the solenoid 27 is no longer able to withhold its plunger against the spring force, at which time tripping occurs. While this form affords a degree of simplicity and lower cost, it lacks the advantage of the previous form which permits use of a standard undervoltage trip" solenoid 27 even though such solenoids are not ordinarily well-suited for operation as part of an extremely accurate time-delay relay system.

While the invention has been described in only two specific embodiments, it will be readily apparent that many modifications thereof may readily be made, and it is therefore intended, by the appended claims, to cover all such modifications as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electric circuit breaker comprising:

a. at least one pair of relatively separable contacts;

b. operating mechanism for operating said contacts between open and closed circuit positions respectively, said operating mechanism including a releasable member and means normally latching said releasable member against movement, said releasable member being releasable to cause automatic opening of said contacts by said mechanism;

c. first current responsive means responsive to relatively low abnormal currents through said circuit breaker;

. means connecting said first current responsive means to said latch means for causing the release of said latch means upon the occurrence of predetermined relatively low overload currents through said circuit breaker;

e. second current responsive means responsive to relatively high overload currents through said circuit breaker;

f. switch means operated by said second current responsive means;

g. electrically operated means connected to said switch means, said electrically operated means comprising means for releasing said latch upon actuation of said electrically operated means;

h. circuit means interconnecting said switch means and said electrically operated means, said circuit means including time-delay means for introducing an accurately controlled short-time delay between the actuation of said switch and the operation of said latch, whereby said circuit breaker may be utilized with other circuit breakers in a system in which circuit breakers operate in a predetermined time sequence.

2. An electric circuit breaker as set forth in claim 1 wherein said circuit breaker also comprises conductor means connecting said first and second current responsive means and electromagnetically penneable shielding means between said conductor means and at least a portion of said second current responsive means.

3. An electric circuit breaker as set forth in claim I wherein said first current responsive means comprises inverse-time characteristic current-responsive means and wherein said second current-responsive means comprises substantially instantaneous acting current-responsive means.

4. An electric circuit interrupter as set forth in claim 3 wherein said circuit breaker also comprises conductor means connecting said first and second current-responsive means and electromagnetically permeable shielding means between said conductor means and at least a portion of said second current responsive means.

5. An electric circuit breaker as set forth in claim 1 wherein said electrically operated means for releasing said latch comprises a solenoid including a winding and a spring-biased plunger normally held in retracted position when said solenoid winding is energized and normally charged capacitance means, said capacitance means providing electrical power for said winding for a short time after the removal of said external power source by opening action of said electric switch.

6. An electric circuit breaker as set forth in claim 1, wherein said electrically operated means for releasing said latch comprises an electric solenoid having a winding and a springbiased plunger biased for movement outwardly of said winding, said plunger when released from said winding being moved by said spring-biasing means to engage said latch and cause the release thereof, said circuit breaker also comprising:

a. an electric relay including a winding and at least one pair of normally open contacts;

b. electrical capacitance means;

c. charging means for charging said capacitance from an external source of electric power;

d. said switch means being connected electrically in series with said capacitance means and said external source of power;

c. said relay winding being connected electrically in series with a predetermined resistance means and said capacitance, whereby said relay is normally energized by said external source of power when said switch means is in closed condition, to maintain said contacts in closed condition, and upon opening of said switch means said relay winding is disconnected from said external source of power and receives power from said capacitance through said resistance, said power lasting a predetermined time to provide a predetermined accurately controlled timedelay between the opening of said switch means and the opening of said relay contact;

f. said relay contact being connected electrically in series with said electrically operated means, whereby said electrically operated means is operated upon opening of said relay contacts to cause release of said circuit breaker latch means.

7. An electric circuit breaker as set forth in claim 6 wherein said circuit also includes additional capacitance and resistance means connected electrically in series to form a series combination, said series combination being connected electrically in parallel with said electrically operated means through said switch means, whereby said second capacitance and resistance combination provides power to said solenoid for a length of time at least as long as the aforesaid time delay provided by said time-delay relay.

8. An electric circuit breaker comprising:

a. a first generally rectangular insulating casing;

b. at least one pair of relatively separable contacts supported in said insulating casing;

c. operating means supported in said insulating casing for operating said contacts between open and closed circuit positions respectively;

d. said operating mechanism including a releasable member releasable to cause automatic opening of said contacts and latch means normally restraining said releasable means in latched position;

e. first current responsive means supported in said first insulating casing, means connecting said first current responsive means to said latch member whereby said first current-responsive means causes release of said latch member upon the occurrence of predetennined relatively low overload currents through said contacts;

f. a second generally rectangular insulating casing having its cross-sectional dimensions substantially equal to said cross-sectional dimensions of said first insulating casing;

g. second current responsive means mounted in said second insulating casing and connected electrically in series with said contacts, said second current-responsive means being responsive to relatively high overload currents through said contact;

h. electrical switch means supported in said second insulating casing for operation by said second current responsive means;

i. an electric solenoid mounted in said first insulating casing and including an armature disposed and arranged to engage said latch member and cause release of said latch member upon the occurrence of predetermined electrical conditions in said solenoid; circuit means interconnecting said electric switch in said second insulating casing to said solenoid in said first insulating casing, said circuit means including time-delay means providing a predetermined accurately controlled time-delay between actuation of said switch means and operation of said solenoid means to cause release of said latch means, whereby said circuit breaker may be used in an electrical system with other circuit breakers to achieve opening in response to fault currents in a predetermined sequence.

9. An electric circuit breaker as set forth in claim 8 wherein said solenoid comprises a winding and a movable armature and means biasing said armature away from said winding and means connecting said armature to said latch means fur causing releasing movement of said latch means upon predetermined movement of said armature in a direction away from said winding. 

1. An electric circuit breaker comprising: a. at least one pair of relatively separable contacts; b. operating mechanism for operating said contacts between open and closed circuit positions respectively, said operating mechanism including a releasable member and means normally latching said releasable member against movement, said releasable member being releasable to cause automatic opening of said contacts by said mechanism; c. first current responsive means responsive to relatively low abnormal currents through said circuit breaker; d. means connecting said first current responsive means to said latch means for causing the release of said latch means upon the occurrence of predetermined relatively low overload currents through said circuit breaker; e. second current responsive means responsive to relatively high overload currents through said circuit breaker; f. switch means operated by said second current responsive means; g. electrically operated means connected to said switch means, said electrically operated means comprising means for releasing said latch upon actuation of said Electrically operated means; h. circuit means interconnecting said switch means and said electrically operated means, said circuit means including timedelay means for introducing an accurately controlled short-time delay between the actuation of said switch and the operation of said latch, whereby said circuit breaker may be utilized with other circuit breakers in a system in which circuit breakers operate in a predetermined time sequence.
 2. An electric circuit breaker as set forth in claim 1 wherein said circuit breaker also comprises conductor means connecting said first and second current responsive means and electromagnetically permeable shielding means between said conductor means and at least a portion of said second current responsive means.
 3. An electric circuit breaker as set forth in claim 1 wherein said first current responsive means comprises inverse-time characteristic current-responsive means and wherein said second current-responsive means comprises substantially instantaneous acting current-responsive means.
 4. An electric circuit interrupter as set forth in claim 3 wherein said circuit breaker also comprises conductor means connecting said first and second current-responsive means and electromagnetically permeable shielding means between said conductor means and at least a portion of said second current responsive means.
 5. An electric circuit breaker as set forth in claim 1 wherein said electrically operated means for releasing said latch comprises a solenoid including a winding and a spring-biased plunger normally held in retracted position when said solenoid winding is energized and normally charged capacitance means, said capacitance means providing electrical power for said winding for a short time after the removal of said external power source by opening action of said electric switch.
 6. An electric circuit breaker as set forth in claim 1, wherein said electrically operated means for releasing said latch comprises an electric solenoid having a winding and a spring-biased plunger biased for movement outwardly of said winding, said plunger when released from said winding being moved by said spring-biasing means to engage said latch and cause the release thereof, said circuit breaker also comprising: a. an electric relay including a winding and at least one pair of normally open contacts; b. electrical capacitance means; c. charging means for charging said capacitance from an external source of electric power; d. said switch means being connected electrically in series with said capacitance means and said external source of power; e. said relay winding being connected electrically in series with a predetermined resistance means and said capacitance, whereby said relay is normally energized by said external source of power when said switch means is in closed condition, to maintain said contacts in closed condition, and upon opening of said switch means said relay winding is disconnected from said external source of power and receives power from said capacitance through said resistance, said power lasting a predetermined time to provide a predetermined accurately controlled time-delay between the opening of said switch means and the opening of said relay contact; f. said relay contact being connected electrically in series with said electrically operated means, whereby said electrically operated means is operated upon opening of said relay contacts to cause release of said circuit breaker latch means.
 7. An electric circuit breaker as set forth in claim 6 wherein said circuit also includes additional capacitance and resistance means connected electrically in series to form a series combination, said series combination being connected electrically in parallel with said electrically operated means through said switch means, whereby said second capacitance and resistance combination provides power to said solenoid for a length of time at least as long as the aforesaid time delay provided by said time-deLay relay.
 8. An electric circuit breaker comprising: a. a first generally rectangular insulating casing; b. at least one pair of relatively separable contacts supported in said insulating casing; c. operating means supported in said insulating casing for operating said contacts between open and closed circuit positions respectively; d. said operating mechanism including a releasable member releasable to cause automatic opening of said contacts and latch means normally restraining said releasable means in latched position; e. first current responsive means supported in said first insulating casing, means connecting said first current responsive means to said latch member whereby said first current-responsive means causes release of said latch member upon the occurrence of predetermined relatively low overload currents through said contacts; f. a second generally rectangular insulating casing having its cross-sectional dimensions substantially equal to said cross-sectional dimensions of said first insulating casing; g. second current responsive means mounted in said second insulating casing and connected electrically in series with said contacts, said second current-responsive means being responsive to relatively high overload currents through said contact; h. electrical switch means supported in said second insulating casing for operation by said second current responsive means; i. an electric solenoid mounted in said first insulating casing and including an armature disposed and arranged to engage said latch member and cause release of said latch member upon the occurrence of predetermined electrical conditions in said solenoid; j. circuit means interconnecting said electric switch in said second insulating casing to said solenoid in said first insulating casing, said circuit means including time-delay means providing a predetermined accurately controlled time-delay between actuation of said switch means and operation of said solenoid means to cause release of said latch means, whereby said circuit breaker may be used in an electrical system with other circuit breakers to achieve opening in response to fault currents in a predetermined sequence.
 9. An electric circuit breaker as set forth in claim 8 wherein said solenoid comprises a winding and a movable armature and means biasing said armature away from said winding and means connecting said armature to said latch means fur causing releasing movement of said latch means upon predetermined movement of said armature in a direction away from said winding. 